This invention relates to filler materials for certain types of overvoltage protection devices that are sometimes known as quantum tunneling varistors.
Transient electromagnetic conditions such as transient voltage and current spikes can damage circuitry or cause errors in their operation. Such electromagnetic transients can occur for a wide variety of reasons. Common sources are electromagnetic pulses (EMP), electrostatic discharges (ESD) and lightning. Electromagnetic pulses can be generated by thermonuclear explosions, high power microwaves or by other high-energy directed devices. Electrostatic discharge transients commonly arise in office, industrial and/or home environments from the accumulation of static charges. An ordinary example is a discharge of static electricity from a person wearing insulating clothing in a carpeted office. Another example is power line switching transients. EMP and ESD transients can generate peak voltage and/or current conditions within a few nanoseconds. Lightning generates peak voltages within a few microseconds. The time required for a transient to reach peak voltage and/or current conditions is referred to as the “rise time”.
Various devices (so-called electrical overstress (EOS) protection devices) are known for protecting electronic circuitry from transient voltage and current conditions that exceed the capacity of the circuits. EOS protection devices generally shield electronic circuitry using two approaches. Devices like fuses and circuit breakers interrupt the connection between the circuit and the transient, thus isolating the circuit from the transient voltage. Such devices are very effective, but their response times are slow in comparison with the rise time of the transient. This results in an “overshoot” condition, where voltages much greater than the protection voltage are temporarily seen before the EOS protection device responds and diverts the transient to ground. This overshoot can cause the circuit to become damaged despite the presence of the EOS protection device. In addition, these devices usually must be replaced or reset after the transient event is over in order to restore the circuit to operation.
Other EOS protection devices operate by diverting the transient current to ground. Varistors are examples of this type of device. A varistor has a property known as “non-linear resistance”, or “NLR”. At low applied voltages, the varistor provides high resistance and acts essentially like an open circuit. Above a certain characteristic voltage threshold (known as a “protection voltage”), the resistance reduces quite substantially. When a transient is experienced, the protection voltage is exceeded and the varistor resistance drops. This allows current to flow through the varistor to ground rather than through the circuits.
A particular type of varistor is known as a metal insulator varistor (MIV). MIVs depend on micron sized, insulator coated particles embedded in a moldable polymeric binder. The insulator coatings and the binder material provide electrical resistance, which is overcome when a protection voltage is exceeded. Coating thicknesses are typically of the order of 5 to 1000 nm.
MIV devices in which the coating is sufficiently thin are known as “quantum tunneling” varistors. These devices are described, for example, in U.S. Pat. Nos. 4,726,991, 4,977,357 and 4,992,333. These devices consist of fine conductor particles in a binder material. In some cases, semiconductor particles are also used. The conductor particles (and semiconductor particles, when present) are separated from each other by distances that are on the order of 100–1000 nm. The material separating the conductor particles is nonconductive. Above a certain protection voltage, current will flow from particle to adjacent conductor particle, through the intervening nonconductive material, via a process known as quantum tunneling. Quantum tunneling behavior is explainable through a probabilistic model of electron behavior, and electrons traverse the intervening nonconductive material by tunneling rather than because their energies exceed the energy barrier imposed by the nonconductive material.
Quantum tunneling varistors offer the possibility of extremely rapid response times that are on the order of the rise times of EMP and ESD transients. This would largely eliminate “overshoot” conditions. In principle, quantum tunneling varistors will provide a highly nonlinear resistance with very high resistance at system operating voltages and low resistance at transient voltage conditions.
The operation of quantum tunneling varistors, as described in U.S. Pat. Nos. 4,726,991, 4,977,357 and 4,992,333, depends on the careful, uniform spacing of the conductor (and semiconductor) particles. The desired quantum tunneling effect is achieved only when these particles are spaced apart at particular distances. The quantum tunneling effect does not occur at all if this spacing is too great. When conductor and semiconductor particles are in contact with each other, a conductive path forms through the varistor even at low voltages. When the spacing is not highly uniform, numerous conductive pathways form at different voltages. As a result, the varistor does not exhibit a clear protection voltage above which resistance drops in a non-linear fashion. Instead, resistance tends to decrease more linearly with increased voltage.
U.S. Pat. Nos. 4,726,991, 4,977,357 and 4,992,333 describe different approaches to obtaining the desired spacing. These can be summarized generally as (1) coating the particles, (2) separating the particles with a non-conductive binder material and (3) separating the particles with fine particles of a non-conductive material. It is very difficult to obtain uniform particle spacing, as this depends on obtaining an almost perfectly uniform distribution of the particles in a binder matrix. Properly coated particles would in principle not require such careful control over particle spacing, but these patents do not describe any method by which effective coatings could be applied. As a result, the coated particle approach is combined with other approaches, such as the use of a nonconductive binder and additional semiconductor particles, to attempt to obtain the desired particle spacing.
Attempts have been made to make coated particles for MIV applications, using a solution chemistry approach. The resultant particles are generally not uniformly coated, nor are the coatings well adhered to the underlying base particle. The non-uniformity leads to statistical variation between the current paths in the varistor material and ultimately to what is called dominant path failure. The poor adhesion of the coating leads to damage during the manufacturing process. The combination of these two problems results in product variability and low production yields.
It is desirable to provide a non-linear resistance material that provides a rapid response time and good nonlinear resistance properties, and can be made easily and reproducibly.